One method of treating aqueous waste material is to pass it vertically through a biological filter, that is a bed of solid material usually in particulate form, which supports micro-organisms. Normally such a filter is used in municipal sewage treatment plants. In such a plant a primary settling step is performed to remove relatively large suspended solids from the sewage. The supernatant liquid is then passed vertically downwards through the filter bed. The action of the micro-organisms retained in the bed reduces the biochemical oxygen demand (BOD) of the sewage. This type of plant is a widely-used alternative to the activated sludge process, according to which activated sludge is added to the sewage which is then passed to and held in a treatment tank. An oxygen containing gas is then introduced into the sewage to stimulate the bacteria in the sludge thereby promoting break-down of the harmful constituents of sewage.
U.S. Pat. No. 3,933,629 relates to making the optimum use of a biological filter. According to that specification there is provided a process for treating a filter bed in which micro-organisms are supported, wherein the micro-organisms are supplied with oxygen by introducing oxygen-enriched gas into water and passing the oxygenated water through the filter bed. The introduction of the oxygen-enriched gas which is substantially pure oxygen or oxygen-enriched air, helps to maintain aerobic conditions in the filter.